1. Field of the Invention
This invention relates to a process for removing water and hydrogen sulfide from gas streams containing predominantly carbon dioxide. More particularly, the invention relates to the selective removal of H.sub.2 O and H.sub.2 S from streams containing large quantities of CO.sub.2 with a particular solvent.
2. Description of the Prior Art
Mixtures of H.sub.2 S with other gases, such as CO.sub.2 and methane, are found in a number of industries. For example, mixtures of H.sub.2 S, CO.sub.2, water, and methane are found as natural gases. It is frequently necessary to remove H.sub.2 S from gas mixtures for the purpose of purifying the gas mixture or recovering the H.sub.2 S or both. For example, it is often necessary to purify a gaseous hydrocarbon stream to produce sweet, dry gas which will not poison certain catalysts and will meet the usual pipeline specifications, and it is sometimes advantageous to recover the H.sub.2 S as a source of elemental sulfur. Furthermore, in the treatment of gas mixtures containing both H.sub.2 S and CO.sub.2 the removal of H.sub.2 S facilitates the subsequent recovery of pure CO.sub.2. In such processes it is frequently advantageous to selectively separate the H.sub.2 S from the other gases comprising the mixture, thus making possible the use of smaller capacity equipment for the subsequent treatment of the separated gas. In particular, in the separation of H.sub.2 S from sour natural gas, economies can be realized by selectively removing the H.sub.2 S with as little as possible of the carbon dioxide.
One process to selectively remove H.sub.2 S from streams containing H.sub.2 S and CO.sub.2 is disclosed in U.S. Pat. No. 3,362,133. In the '133 patent the gas feed is contacted with a liquid solvent comprising a normally liquid dialkyl ether of a polyalkylene glycol. A specific solvent employed therein is the dimethyl ether of a polyethylene glycol such as diethylene glycol or triethylene glycol. This process is practiced commercially and is known as the Selexol process. The Selexol process has certain advantages for removing small amounts of H.sub.2 S and CO.sub.2 from other gases as a result of its high solvency for H.sub.2 S and CO.sub.2 and relatively poor solvency for other gases such as methane. These advantages included notably the use of a relatively low solvent flow. It may also be used to remove H.sub.2 S from streams high in CO.sub.2 content. However, this process has certain disadvantages for removing H.sub.2 S from gases high in CO.sub.2 in the absence of a substantial level of methane. In this case, particularly at high CO.sub.2 pressures, the solvent may be totally miscible with the gas and consequently its use gives rise to no separation unless the pressure is lowered to a level at which the system is not miscible in all proportions. The latter requires additional compression of the product CO.sub.2 to the high (supercritical) pressures normally used for transport by pipeline.
A new process has been found for selectively removing H.sub.2 S from gas streams, containing predominantly CO.sub.2 and H.sub.2 S, at high pressures without excessive loss of solvent, as with the Selexol process.